The tumor necrosis factor (TNF) receptor family has several members whose signaling can induce tumor cell death by necrosis or apoptosis (programmed cell death). The ligands TNF and lymphotoxin-.alpha. (LT-.alpha.; formerly called TNF-.beta.) bind to and activate TNF receptors (p60 and p80; herein called "TNF-R"). TNF-R signaling initiates general immune responses to infection or stress in normal cells, but is cytotoxic to cells with transformed phenotypes or to tumor cells. TNF-R signaling can selectively lyse tumor cells and virus-infected cells. The cytotoxic effects of TNF-R signaling on tumor cells are enhanced by interferon-.gamma.) and by a variety of conventional chemotheropeutic agents.
It would be useful to take advantage of the anti-proliferative or cytotoxic activities induced by TNF-R signaling in tumor cells for therapeutic purposes. However, TNF-R activation has pleiotropic effects on a variety of immunoregulatory responses including the initiation of proinflammatory cascades. Thus it has not been possible to direct the cytotoxic effects of TNF-R signaling to tumor cells without co-stimulating inflammatory responses which lead to general toxicity in humans.
Similarly, stimulation of another TNF-related receptor called the Fas receptor (FasR) can trigger cytotoxicity by programmed cell death in a variety of both tumor and non-tumor cell types. However, FasR activation has been shown to cause rapid liver necrosis, thus precluding its therapeutic application in humans.
Recently, another receptor in the TNF family called the LT-.beta. receptor (LT-.beta.-R) was identified (Crowe et al., Science, 264, pp. 707-10 (1994). The LT-.beta.-R binds heteromeric lymphotoxin complexes (LT-.alpha./.beta.) which comprise LT-.alpha. subunits in association with another TNF-related polypeptide called lymphotoxin-.beta. (LT-.beta.). These LT-.alpha./.beta. complexes are membrane-associated and most have a LT-.alpha.1/.beta.2 stoichiometry (Browning et al., Cell, 72, pp. 847-56 (1993); Browning et al., J. Immunol., 154, pp. 33-46 (1995)).
By analogy to TNF-R and other TNF-like receptors, the activation of LT-.beta.-R signaling is thought to occur when multiple receptors on the cell surface are brought into close proximity (Crowe et al., Science, 264, pp. 707-10 (1994)). This process is referred to as receptor clustering. The TNF and LT ligands are multivalent complexes which can simultaneously bind to and thus aggregate more than one receptor. Receptor clustering as a means for receptor activation in other systems has been well-documented, especially for receptor tyrosine kinases (Ullrich and Schlessinger, Cell, 61, pp. 203-212 (1990); Kolanus et al., Cell, 74, pp. 171-83 (1993)). Accordingly, administering LT-.alpha.1/.beta.2 ligands and/or LT-.beta.-R activating agents which can induce the clustering and downstream signaling of LT-.beta.-R molecules on the surface of target tumor cells would be useful for directly stimulating the LT-.beta.-R pathway in these cells.
Signaling by LT-.beta.-R, like TNF-R, can activate pathways that lead to cytotoxicity and cell death in tumor cells. Importantly, LT-.alpha.1/.beta.2 ligands do not bind to TNF-R with any significant affinity. For this reason, directed LT-.beta.-R activation in tumor cells would trigger cytotoxicity in those cells without stimulating the inflammatory pathways associated with TNF-R activation. Treatment with LT-.alpha.1/.beta.2 and/or other LT-.beta.-R activating agents would thus be useful for treating or reducing the advancement, severity of effects of tumorigenic cells (neoplasia) while overcoming the potent side effect problems which have been encountered when TNF-R or FasR activation has been tried as an anti-tumor treatment.